It is the other way around.
It is a volatile market because it is potentially not down to guess work.
Most betting comes down to guess work because (in theory) no one can know the result of a match, it is a guess work all round. The bookies price in their odds and take home a good amount of money no matter the result.
With a manager appointment, at some point someone is going to know the result before the bookies do, thus taking the guess work out of it. To cover themselves bookmakers give crap odds, adjust the odds dramatically anytime they get a bet in, restrict the maximum amount that can be bet and suspend betting anytime a decent volume of bets come in on one person. It makes for a much more volatile market.